The present invention relates to a method for cutting limp sheet material with closed loop control. More particularly, the present invention relates to an automatically controlled cutting machine having a rigid, cantilever-mounted knife blade which advances along a cutting path through the sheet material and which is oriented slightly out of a position of tangency by means of a lateral load sensor to oppose loads that bend the blade out of its desired cutting position.
U.S. Pat. No. 4,133,235 issued Jan. 9, 1979 and having the same assignee as the present invention discloses a method and apparatus for cutting limp sheet material for garments, upholstery and other items. The disclosed machine utilizes a reciprocated knife blade that is mounted in cantilever fashion from a tool carriage and which is advanced along a cutting path under programmed control in cutting relationship with a stack or layup of the sheet material. During the cutting operation the depending end of the knife blade penetrates through the stack of material, and loads developed by the interaction of the blade and material operate on the blade. Lateral loads cause the depending end of the knife blade to bend which produces cutting errors regardless of the accuracy with which the upper end of the blade has been positioned by drive motors moving the tool carriage.
To correct the cutting error created by lateral loads, a sensor measures the loads applied to the blade, and through a feedback circuit orients or yaws the blade slightly out at a position tangent to the cutting path and toward the side of the cutting path from which an unbalanced load is applied. The reorientation as the knife blade advances along the cutting path has the effect of opposing the lateral loads and results in more accurate cutting of the limp sheet material.
It has been found that at high cutting rates, that is when the cutting blade and the sheet material are fed relative to one another at high speeds, the loads applied to the cutting blade reach higher levels than at lower cutting speeds, and as a consequence the corrective orientations of the blade are too severe. Under these circumstances the blade is overdriven and a wavy line of cut is generated along cutting paths which should otherwise be straight or have a smooth, gradual curve.
It has additionally been determined that although a reduction in the amount of corrective orientation eliminates the wavy cutting along high speed sections of the cutting path, a corresponding deficiency develops in other critical cutting situations when the corrective orientation is needed at low speeds. For example, at the tangency of two cutting paths, a relatively large amount of yawing is required to prevent the cutting blade from jumping into the adjacent cutting path when the second cut is being made through the point of tangency.
Accordingly,it has been determined that the variation in lateral force levels experienced at different cutting speeds interferes with closed loop control of blade orientation by means of a lateral load sensor. It is accordingly a general object of the present invention to overcome this problem and to obtain high accuracy cutting with a knife blade under a wide variety of cutting circumstances. More particularly, it is an object of the present invention to obtain more accurate cutting over a broad range of cutting speeds.